FIG. 1 is an illustration of an exemplary propulsive efficiency graph showing exemplary installed propulsive efficiency vs. Mach number curves 102/104/106/108 for several types of propulsion systems. As shown in FIG. 1, generally, slower aircraft (in a range from Mach 0.4 and 0.6) have more efficient propulsion options. Propellers with no outer ducts, nacelles, or shrouds can be very efficient, converting close to 90% of the engine's horsepower into thrust. As shown by the curve 104, at flight speeds above Mach 0.7, propellers become less practical because a speed of flow passing over rotating propeller blades approaches or exceeds Mach 1.0, where there is increased aerodynamic drag. High speed propellers have additional disadvantages with respect to noise, weight, complexity, and cost.
Therefore, as shown on the curve 108, turbofan engines are generally used on modern transports because they operate at flight speeds generally between Mach 0.75 and Mach 0.85 that are too fast for practical propellers. Turbofan engines generally have greater parasitic drag and lower disk area in comparison to a turboprop; however, turbofans generally convert only about 70% of shaft horsepower into thrust.
Fuel contributes to approximately half of direct operating cost for an airliner, and the future may bring higher fuel prices and increased environmental considerations such as carbon taxes. With increasing fuel prices and competition among aircraft manufacturers, there is a pressing need for more efficient aircraft.